That's pretty intensive pruning to cut each tip into a point.. Most have more of an arching growth... but really, they need more room than that.. Pruning that hard is a crime to me. It's just not the best plant for that application.
If you want something that gives that vertical effect, and have a *contained* space (It runs), a real architectural plant that looks especially good with modern architecture is Equisetum hymenale.. I think it would serve you better for that type of look than the NZ Flax... which wants to be 6' around...